Under the situation where the function of an information communication apparatus such as a wireless terminal is being enhanced, increase of the adaptive information amount of an information apparatus is essential in provision of services to meet needs of users. In a memory for a wireless terminal, performances such as reduced size, large capacity, and low power consumption are requested. Currently, semiconductor memories of DRAM (Dynamic Random Access Memory), SRAM (Static Random Access Memory), and SDRAM (Static Dynamic Random Access Memory) are used. A DRAM is a storage device which requires a record holding operation, and has a problem of increased power consumption.
At present, research and development of reduction of power consumption are advanced. A technique of incorporating a function of suppressing a refresh operation to a minimum level to reduce the power consumption of a DRAM for a personal computer to 1/10 has been developed. A memory is provided with PASR (Partial Array Self Refresh) which is a function of dividing the memory into blocks and refreshing only a required block, and TCSR (Temperature Compensated Self Refresh) which is a function of automatically determining an optimum refresh time in accordance with the temperature to extend the average refresh time. At a clock frequency of 100 MHz, the data transfer rate is 400 Mbyte/s, and the power source voltage is 1.8 V which is usually used in a portable telephone (Elpida Memory, May 2004).
Furthermore, also research and development of an SDRAM which is to be used in a wireless terminal, and which combines high speed of an SRAM and large capacity of a DRAM are conducted. In such an SDRAM, the DDR (Double Data Rate) technique is applied, and techniques for reducing power consumption such as the data transfer rate of 1.1 Gbyte/s, 256 MB, PASR, and TCSR are employed. It is a technique directed to a use of displaying of a three-dimensional motion picture in a portable apparatus such as a portable telephone, or a PDA (Samsung Electronics, May 2004). In a semiconductor memory element, recording of 1, 0 bits of information is performed by means of accumulation or discharging of charges in a capacitor. Temporal deterioration of information due to reduction of the amount accumulated charges occurs in a volatile memory. When paths along which charges escape are completely interrupted, this phenomenon can be avoided. In electrical switching by transistors, however, it is difficult to completely interrupt electric conductive paths, and the phenomenon inevitably occurs. Therefore, it is required to perform the record holding operation at constant time intervals. The operation causes the power consumption to be increased.
Accordingly, research and development of a nonvolatile memory element which is to be replaced with a conventional semiconductor memory element are conducted. An MRAM (Magnetic Random Access Memory), and an FeRAM (Ferroelectric Random Access Memory) are configured so that a magnetic material or a dielectric material is used as a memory cell, and magnetic polarization or electric polarization is used as a record data which can be semipermanently held. This technique does not require a record holding operation, and hence is expected as a memory element technique which can realize low power consumption. Therefore, such a memory is contemplated to be applied to a field in which reduction of power consumption is important, such as a portable terminal.
As a device which has high affinity with a conventional semiconductor process, and in which interruption of electric conductive paths is enabled, an electromechanical switch which is produced by the microelectromechanical systems (MEMS) technique is being vigorously researched and developed. Originally, the research field is mainly directed to an RF-MEMS switch which switches over a propagation path for a high-frequency signal in a wireless communication terminal. An RF-MEMS switch is a switch in which a micro movable electrode is moved to mechanically switch over a signal transmission path. The switch has an advantage that high-frequency characteristics such as very low loss and high isolation are excellent.
Furthermore, the switch can be produced by a process which has high affinity with an RF-IC. Therefore, the switch can be incorporated in an RF-IC, and is expected as a technique which significantly contributes to size reduction of a radio component.
As a switch having a size of about several hundreds of μm, conventionally, a switch described in Non-patent Reference 2 has been known. In the switch, a signal line through which a high-frequency signal is transmitted is formed on a membrane, and a control electrode is disposed just below the signal line. When a DC potential is applied to the control electrode, the membrane is attracted toward the control electrode by an electrostatic attractive force to deflect, and makes contact with a ground electrode formed on a substrate, whereby the signal line formed on the membrane is set to a short-circuited state, a signal flowing through the signal line is attenuated, and the line is interrupted. By contrast, when a DC potential is not applied to the control electrode, the membrane does not deflect, and a signal flowing through the signal line on the membrane is not lost from the ground electrode, and passes through the switch.
As a memory element to which an electromechanical switch is applied, there is an electromechanical switch disclosed in Patent Reference 1. Patent Reference 1 discloses a structure in which a capacitor to perform accumulation is selected by an electromechanical switch, and accumulated charges are gradually consumed for a constant time period by a light emitting element. The light emitting element has a memory function of a constant time period.
Patent Reference 1: JP-A-2002-366058
Non-patent Reference 1: Y. Asao et al., in Int. Electron Device Meeting Tech. Dig., December 2004.
Non-patent Reference 2: J. B. Muldavin and G. M. Rebeiz, IEEE Microwave Wireless Compon. Lett., vol. 11, pp. 334-336, August 2001.